Systems for preventing locking of the vehicle wheels of vehicles (anti-lock braking systems, ABS) are known.
The control parameters of such an ABS system are set in such a way that different roadway conditions, i.e. both uneven roadways and gravel roadways, are taken into account in order to prevent a deterioration in the effect of the ABS system. The reason for this is that unevenesses in the roadway represent a disturbance variable in the ABS control circuit, since the impacts effected on the vehicle wheel by such unevenesses can result in a short-term occurrence of slip before the actual locking pressure level is reached.
In order to take account of the road conditions, a so-called rough roadway detection function is integrated into the ABS system in order to adjust controlled variables of the ABS system upon detection of undulating or bumpy or rough roadway surfaces or gravel roadways, which are referred to collectively as “poor roadways”, thereby making it possible, e.g., to increase the intervention thresholds of the control algorithms in order to increase the slip values.
A method for determining road conditions is known from the generic document DE 199 10 099 A1, which is incorporated by reference, in which characteristic variables are derived from the dynamic behavior of the vehicle wheels, which is directly influenced exclusively by the roadway, and are evaluated. In this known method, the rotational behavior of the vehicle wheels or the wheel speeds are differentiated, as characteristic variables, with respect to time and poor roadway conditions are detected by comparing the arithmetic mean with the absolute mean, both positive and negative changes in speed being used as wheel acceleration values.
Furthermore, document EP 1 294 599 B1, which is incorporated by reference, discloses a method for improving the control behavior of a braking system (ABS) having anti-lock braking control when imperfections occur in the roadway, such as transverse grooves, potholes, ridges, etc., in which method the rotational behavior of the wheels is evaluated in order to perform anti-lock braking control and to detect imperfections in the roadway when the anti-lock braking control starts, subsequent to partial braking, in that a brief brake pressure reduction phase with a pronounced reduction in brake pressure at a wheel as a consequence of strong wheel deceleration or wheel acceleration, followed by strong re-acceleration of the wheel, is interpreted as being an imperfection in the roadway either if the quotient between the duration of the reduction in pressure and the total duration of the pressure reduction phase multiplied by the re-acceleration of the wheel or the peak value of the re-acceleration or if the re-acceleration change or the peak value of the re-acceleration change exceed prescribed limit values, and in that a rise in brake pressure which is increased compared to the standard ABS braking control is caused as a consequence of the detection of the imperfection in the roadway.
Finally, DE 100 20 215 A1, which is incorporated by reference, also describes a method for detecting a gravel roadway or a similar roadway with a higher slip requirement, in which the vibration behavior of the individual vehicle wheels is detected at the driven axle, and is evaluated, and the driving situation of a gravel roadway is detected when the wheel acceleration exceeds a prescribed wheel acceleration limit value at at least two vehicle wheels and when the at least two vehicle wheels exhibit a specific vibration behavior which is characteristic of a gravel roadway.